The present invention relates to an improvement in an image information processing method for processing hafltone images in photographs, pictures and the like and, more particularly, to a tone information processing method for providing bit distribution pattern data representative of a halftone image from tone data.
Prior art tone information processing methods include a fixed density pattern type method which uses a plurality of halftone matrix patterns each comprising a predetermined number of bits and having record (display) information bits distributed in a predetermined mode. One of the matrix patterns associated with a particular density indicated by tone data is specified (selected) and, then, its bit information is developed in a page memory or a buffer memory while being matched to recording (displaying) positions of the tone data. The problem with this method is that since a density usually has correspondence with a number of pixels (bits) in a matrix, an increase in the number of tones to be rendered causes the matrix itself to become large-sized. Such is reflected by a decrease in resolution because one matrix is extracted by one tone data. While a smaller matrix is successful to enhance the resolution, it gives rise to another problem that the number of available tones is reduced to in turn lower the smoothness in halftone representation.
To overcome the above problem, there has been proposed a method which provides image information by changing the size of a matrix depending upon the roughness of an image (e.g. Japanese Patent Laid-Open Publication No. 57-159173/1982). This approach is impractical, however, in view of the required precision and difficulty in identifying a roughness of an image and the need for numerous kinds of different matrices and, therefore, a considerable amount of data fixedly stored memories.
In accordance with another prior art method, a plurality of (four) different kinds of small matrix patterns (4 pixels:five tones) are combined to define one large matrix (4 kinds.times.4 pixels=16 pixels:4 kinds.times.5 tones=20 tones) (e.g. Japanese Patent Laid-Open Publication No. 57-159174/1982). The small matrix patterns are fixed in position within the large matrix pattern kind by kind. Tone data are allocated to specific ones of the submatrix patterns based on their order of arrangement in an image, thereby producing image information on a small matrix pattern basis. Such a method provides image information in one large matrix pattern as composite information of a plurality of (four) small matrix patterns. Stated another way, one large matrix pattern is specified by a plurality of (four) tone data.
However, the small and large matrix patterns scheme discussed above simply results in the fact that a plurality of (four) kinds of small matrix patterns each determine image information associated with a large matrix, that is, a plurality of (four) tone data determine a large matrix pattern. In effect, the resolution and the smoothness of gradation are dependent upon the small matrix patterns, the large matrix little contributing to an improvement in smoothness of gradation. Specifically, since a small matrix pattern of one kind appears at a predetermined interval (every two small matrix patterns), an irregular or regular pattern develops in a reproduced image as has been pointed out as a drawback of a dither method. In addition, the smoothness of gradation achievable with the large matrix pattern is limited because the matrix resembles a matrix pattern in accordance with the prior art dither method in which a plurality of regions are regarded as a single division.
Meanwhile, it has been proposed to utilize a plurality of groups of matrix patterns having different density characteristics in order to record halftone images with different density characteristics. Such an approach is not fully acceptable, however, because the attainable number of tones in each group is not more than the number of dots which are included in a single matrix pattern. That is, even if groups with several different characteristics are employed, they fail to set up sufficiently smooth gradation or delicate density characteristics.